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INTRODUCTION — Precipitation of crystals of calcium pyrophosphate dihydrate (CPP) in connective tissues may be associated with several clinical syndromes, but is sometimes asymptomatic. The consequences of CPP deposition include acute inflammatory arthritis, inflammatory and degenerative chronic arthropathies, and radiographic cartilage calcification and constitute the spectrum of calcium pyrophosphate crystal deposition (CPPD) disease [1-3].
The clinical manifestations and diagnosis of CPPD disease are discussed here. The pathogenesis and etiology of this disorder and the treatment of CPPD diseases are discussed separately. (See "Pathogenesis and etiology of calcium pyrophosphate crystal deposition (CPPD) disease" and "Treatment of calcium pyrophosphate crystal deposition (CPPD) disease".)
TERMINOLOGY — The names traditionally used for calcium pyrophosphate dihydrate (CPP) crystal deposition (CPPD) diseases include pseudogout, chondrocalcinosis, and pyrophosphate arthropathy. Based upon a review of the relevant literature, however, a European League Against Rheumatism (EULAR) consensus panel suggested alternative terminology (see 'Clinical manifestations' below); the panel also reviewed diagnostic approaches to these conditions and the evidence supporting these approaches . In view of its wider acceptance in the literature since the introduction of this terminology in 2011, we will place primary emphasis here on the EULAR task force terminology , in which the term "calcium pyrophosphate crystal deposition" (abbreviated as "CPPD") is proposed as the umbrella term for all instances of calcium pyrophosphate crystal occurrence.
Nevertheless, despite their limitations, the clinical syndromes and findings implied by the traditional terms for CPPD disease are likely to be retained to a greater or lesser extent by some clinicians [5,6]. Familiarity with the older nomenclature may, in addition, be useful in instances where literature searches using only EULAR task force terms overlook citations indexed under the traditional terms. These include:
●Pseudogout – Pseudogout accurately describes acute attacks of CPPD-induced synovitis, which clinically resemble acute attacks of urate gout. However, the majority of individuals with CPPD never experience such episodes, and the range of clinical events characterizing gout and CPPD disease extend well beyond those that characterize acute gouty arthritis. For these reasons, the EULAR consensus panel prefers the term "acute calcium pyrophosphate (CPP) crystal arthritis" instead of pseudogout.
Notably, however, the use of the term "pseudogout" has also been used more broadly to direct attention to the shared clinical features of the two major crystal-induced arthritides (gout and pseudogout) that are seen much less commonly in other joint diseases, such as rheumatoid arthritis (RA) or spondyloarthritis. These features include recurrent, usually monoarticular, flares of inflammatory arthritis that are self-limited in duration and most often resolve completely. Retention of the diagnostically useful concept of a shared clinical profile of crystal-induced arthritis has merit, as does the mandate to distinguish gout from pseudogout with respect to therapeutic approaches. Thus, use of the term "pseudogout" for acute CPP crystal arthritis will likely persist.
●Chondrocalcinosis – Chondrocalcinosis refers to radiographic calcification in hyaline and/or fibrocartilage (image 1). It is commonly present in patients with CPP crystal deposition disease but is neither absolutely specific for CPPD nor universal among affected patients. The EULAR panel designates this finding as "cartilage calcification (CC)."
●Pyrophosphate arthropathy – Pyrophosphate arthropathy is the term sometimes used for the joint diseases or radiographic abnormalities accompanying CPP crystal deposition. Although this name has little justification in terms of the pathologic anatomy of the disorder, evidence of aberrant inorganic pyrophosphate metabolism in the pathogenesis of CPPD disease may legitimize use of this term. (See "Pathogenesis and etiology of calcium pyrophosphate crystal deposition (CPPD) disease".)
EPIDEMIOLOGY — Calcium pyrophosphate crystal deposition (CPPD) disease has been estimated to affect 4 to 7 percent of the adult populations of Europe and the United States [7,8], especially among persons of advanced age. However, an estimate of the prevalence of clinically significant CPPD disease has been more difficult to attain, in large part because the available prevalence estimates have relied primarily upon radiographically detected cartilage calcification rather than clinical evaluation, and also because prevalence data concerning patients less than about 60 years of age are not available.
The average age at diagnosis of CPPD disease in one study was 72 years . Radiographic surveys have demonstrated an age-related increase in the prevalence of cartilage calcification [9,10]. This was illustrated in a report that supplemented radiographs of the knees with radiographs of the hands, wrists, and pelvis . The prevalence of radiographic calcium pyrophosphate deposition according to age, among 100 consecutive patients admitted to an acute geriatric unit, was:
●65 to 74 years – 15 percent
●75 to 84 years – 36 percent
●>84 years – Almost 50 percent
The gender distribution of CPPD disease has differed among large series [3,11,12], but no major gender predominance appears likely. Attacks of acute arthritis may occur more frequently in men, while typical osteoarthritis (OA) with calcium pyrophosphate crystal deposition or the atypical pattern of OA characteristic of CPPD are more common in women.
CLINICAL MANIFESTATIONS — The majority of individuals with calcium pyrophosphate dihydrate (CPP) crystal deposition (CPPD) disease are asymptomatic with respect to joint involvement, and there is considerable diversity in the patterns of joint disease among those who develop symptoms. A clinical classification of CPPD disease emerged from the studies of McCarty  and colleagues [2,3], which drew particular attention to the capacity for the clinical manifestations of this disorder to mimic virtually any type of arthritis, including gout, rheumatoid arthritis (RA), osteoarthritis (OA), and neuropathic joint disease. This clinical classification (in parentheses below) relates to the nomenclature proposed by the European League Against Rheumatism (EULAR) task force  as follows:
●Asymptomatic CPPD disease ("asymptomatic CPPD") (see 'Asymptomatic CPPD disease' below)
●Acute CPP crystal arthritis ("pseudogout") (see 'Acute CPP crystal arthritis' below)
●Chronic CPP crystal inflammatory arthritis ("pseudo-RA") (see 'Chronic CPP crystal inflammatory arthritis' below)
●OA with CPPD, with or without superimposed acute attacks ("pseudo-OA") (see 'Osteoarthritis with CPPD' below)
●Severe joint degeneration (pseudo-neuropathic joint disease) (see 'Severe joint degeneration' below)
Asymptomatic CPPD disease — Most joints in which CPP crystal deposition is readily apparent on radiographs are asymptomatic, even among patients in whom acute or chronic clinical manifestations of CPPD disease in one or several other joints have occurred. However, patients with apparent asymptomatic CPPD may be found to have manifestations of an arthritic disorder upon close questioning. As an example, in one series of older patients with radiographic but ostensibly asymptomatic CPP crystal deposition, a higher frequency of wrist complaints and genu varus deformity was reported by questionnaire and detected by examination, respectively, than in a control group of similar age but without radiographic chondrocalcinosis .
Acute CPP crystal arthritis — "Acute CPP crystal arthritis" is characterized by self-limited acute or subacute attacks of arthritis involving only one or several extremity joints . The traditional term pseudogout underlines the usually close resemblance of these attacks to those of urate gout, in the accompanying symptoms and signs of severe acute inflammation (intense pain, redness, warmth, swelling, and joint disability), and in the occasional occurrence of synchronous inflammation of several adjacent joints (cluster attacks) or, conversely, petite attacks (which are minimally painful episodes of joint warmth and swelling).
The knee is affected in over 50 percent of all acute attacks of acute CPP crystal arthritis, while the first metatarsophalangeal (MTP) joint is the most frequently affected in urate gout. Other joints typically affected in acute CPP crystal arthritis include wrists, shoulders, ankles, feet, and elbows. Initial episodes of acute CPP crystal arthritis may persist longer before remitting than the one or two weeks commonly encountered in urate gout, and an upper extremity site of inflammation (wrist, elbow, shoulder) for a first attack should raise suspicion for acute CPP crystal arthritis .
Trauma, surgery, or severe medical illness often provoke acute attacks. In particular, flares of acute CPP crystal arthritis after parathyroidectomy have been observed ; these episodes may be related to abrupt reduction in serum calcium and magnesium levels during postoperative hypoparathyroidism (see "Hungry bone syndrome following parathyroidectomy"). Such reduction may cause partial dissolution of crystals with subsequent release from the cartilage matrix into the joint fluid, allowing phagocytosis and the phlogistic response of inflammatory cells.
Intraarticular hyaluronate injection and treatment with pamidronate or granulocyte-macrophage colony-stimulating factor (GM-CSF) [15-17] have also been reported to precipitate acute attacks of pseudogout. However, patients affected with CPPD who experience acute joint inflammation following hyaluronate injection do not demonstrate crystals in their joint fluids .
In our experience, systemic findings and laboratory abnormalities, such as fever, leukocytosis with a left shift in the differential count, and an elevated sedimentation rate and serum C-reactive protein (CRP), occur during attacks of acute CPP crystal arthritis in a small minority of affected patients, and the frequency of each is probably less than in acute gouty arthritis. However, a review of 50 cases of acute CPP crystal arthritis, many of which were polyarticular attacks, described low-grade fevers in 50 percent and frequently elevated sedimentation rates . In some cases, systemic features accompanying polyarticular acute CPP crystal arthritis are quite prominent and may suggest pyogenic arthritis, osteomyelitis, and/or systemic sepsis. (See 'Chronic CPP crystal inflammatory arthritis' below.)
Rarely, and usually after several acute arthritic flares, palpable and visible masses of CPP crystals, resembling gouty tophi, accumulate in synovium and adjacent joint structures, and may lead to locally destructive and compressive symptoms.
Chronic CPP crystal arthritis
Chronic CPP crystal inflammatory arthritis — The term pseudo-rheumatoid arthritis (pseudo-RA) was applied to a nonerosive, inflammatory arthritis in which CPP crystals were demonstrable in joint fluid . This presentation of CPPD disease resembles RA in several respects, including the presence of significant morning stiffness, fatigue, synovial thickening, localized edema, and restricted joint motion due either to active inflammation or to flexion contracture.
Typically, the chronic inflammatory arthritis of CPPD disease involves multiple joints, frequently involving peripheral joints of the upper and lower extremities, including the wrists and metacarpophalangeal (MCP) joints, as well as the knees and elbows, in a symmetric or nearly symmetric pattern. Articular inflammation may last up to several months, and inflammation in affected joints tends to wax and wane independently of one another, in distinction to RA, where synchronous flare and remission is the rule.
Chronic CPP crystal inflammatory arthritis occurs in 5 percent or less of patients with symptomatic CPPD disease. A rare subtype of chronic CPP crystal inflammatory arthritis, occurring most often in older adult patients during an acute polyarticular attack, is characterized by prominent systemic features, such as leukocytosis, fever, and mental confusion, closely mimicking systemic sepsis . In such patients, the delirium is reported to resolve with resolution of the acute polyarticular flare.
Osteoarthritis with CPPD — This is the most prevalent form of symptomatic CPPD disease; for example, 20 percent of unselected patients examined at total knee joint replacement for OA showed CPP crystals in synovial fluid samples . Approximately 50 percent of patients with symptomatic CPP crystal deposition disease show progressive joint degeneration, usually involving multiple joints. This pattern of disease has historically been referred to as "pseudo-OA" because of its resemblance to OA occurring in the absence of CPPD. In about one-half of such patients, episodes of acute inflammatory arthritis typical of pseudogout punctuate the course. In the remainder, joint degeneration proceeds by a process more typical of classical OA. (See "Clinical manifestations and diagnosis of osteoarthritis".)
The most commonly affected joints in this form of CPPD disease are the knees, followed by the wrists, MCP joints, hips, shoulders, elbows, and spine. Although a symmetric pattern of joint involvement is frequent, unilateral or more severe degenerative change on one side is not unusual. Findings on clinical examination of individual joints do not typically differ from those observed in OA, which include asymmetric bony enlargement, tenderness, effusions, crepitus, and restricted joint motion. Patients with OA with CPPD may also exhibit contractures of involved joints and valgus deformities of the knees.
When the arthritic process occurs in joints typical for OA (such as the interphalangeal joints of the hands, the first carpometacarpal joints, the knees, or the bunion joints), even radiograph or joint fluid evidence of CPPD does not permit an unequivocal diagnosis. However, an etiologic or accelerating role for CPP crystal deposition in joint degeneration seems likely when radiographic calcification is present early in the course of degeneration, particularly if the degenerative changes also involve joints atypical for OA (wrists, MCP joints, elbows, and shoulders) in the absence of a preceding history of joint trauma or of vocational or avocational stress.
As cartilage is lost during the course of progressive joint degeneration, previously apparent cartilage calcification may become increasingly difficult or impossible to detect radiographically, thereby obscuring the diagnosis. The radiographic features of CPPD are described. (See 'Imaging findings' below.)
Severe joint degeneration — A number of reports have documented CPP crystal deposition in association with severe joint degeneration which closely resembles neuropathic arthropathy [21-23]. Neuropathic arthropathy is characterized by severe joint degeneration and disruption occurring in the course of neurologic disorders leading to joint denervation; the affected joint is often called a Charcot joint. Underlying disorders associated with Charcot joints include diabetes mellitus (most common), tabes dorsalis, and syringomyelia. (See "Diabetic neuropathic arthropathy".)
In contrast to neuropathic arthropathy, neurologic function is typically normal in severe joint degeneration associated with CPP crystal deposition, prompting use of the term "pseudo-neuropathic joint disease" for this relationship, although an underlying neurologic impairment (typically tabes dorsalis) has, in some instances, been demonstrated [22,23]. The term "pseudo-neuropathic" is intended to convey the view that, when CPP crystal deposition is associated with certain neurologic deficits, the presence of crystals amplifies the destructive consequences of joint denervation.
Spinal involvement — CPP crystal deposition in and about the spine has been associated with a number of clinical manifestations, including spine stiffness, sometimes associated with bony ankylosis, which can resemble the spinal changes of ankylosing spondylitis or diffuse idiopathic skeletal hyperostosis (DISH). Such symptoms have been most commonly encountered in familial CPPD disease . In addition, crystal deposition in the ligamentum flavum at the cervical spine level or in the posterior longitudinal ligament at lower levels of the spine may lead to spinal cord compression syndromes or to symptoms either of acute nerve compression or of chronic spinal stenosis [25-27].
The crowned dens syndrome (CDS) is a rare, but important to recognize, syndrome, which is characterized by severe acute or recurrent axial neck pain, neck and shoulder girdle stiffness, and associated fever; elevated inflammatory markers (C-reactive protein; erythrocyte sedimentation rate, and CPP or calcium hydroxyapatite crystal deposition demonstrable on computed tomography [CT] in and around the atlanto-axial articulation) [28,29]. The importance of identification of the crystal deposition basis of CDS lies both in the resemblance of its symptoms and signs to those of polymyalgia rheumatica, giant cell arteritis, or, less frequently, meningitis, cervical discitis, or inflammatory spondyloarthritis; and in the usually favorable response of CDS clinical features to treatment with nonsteroidal antiinflammatory drugs (NSAIDs) or colchicine.
Other manifestations — CPP crystal deposition occurring in the wake of trauma or prior surgery may result in localized inflammation or degeneration, most commonly in the knee or in the lower or lumbar spine. In addition, CPP crystal deposition in bursae, ligaments, and tendons may be sufficient to cause local nerve compression, as in the carpal tunnel .
SYNOVIAL FLUID FINDINGS — The most salient finding on synovial fluid analysis in calcium pyrophosphate dihydrate crystal (CPP) deposition (CPPD) disease is the presence of positively birefringent CPP crystals by compensated polarized light microscopy. In inflamed joints during an attack of acute CPP crystal arthritis, phagocytosed crystals within polymorphonuclear leukocytes are virtually always present (picture 1). However, certain characteristics of CPP crystals and several technical factors may affect crystal detection.
Total synovial fluid leukocyte concentration in an acute attack is typically 15,000 to 30,000 per mm3, 90 percent of which are neutrophils. In chronically symptomatic joints, crystals are often found extracellularly and cell counts are typically lower.
The abundance of CPP crystals in the synovial fluid is generally related to the degree of clinically apparent inflammation ; however, large discrepancies between these variables can occur. The proportion of intracellular crystals correlates roughly with the level of inflammation (as reflected clinically and by the synovial fluid neutrophil concentration).
CPP crystals differ from the needle-shaped, strongly and negatively birefringent monosodium urate crystals in acute gouty arthritis (picture 2); CPP crystals are:
●More difficult to detect
●Smaller (0.5 to 10 microns)
●Weakly and positively birefringent or not birefringent at all
●More polymorphic with rod-shaped and cuboid crystals in addition to the usual rhomboidal form
In some instances, CPP crystals are too small to be readily visualized; for example, CPP crystals are sometimes smaller than can be resolved even by 1000-fold magnification with the aid of phase contrast microscopy .
Patients with CPP crystals in a synovial fluid sample may also have the simultaneous presence of monosodium urate crystals. The coexistence of urate and CPP crystals in a single inflammatory effusion is neither uncommon nor unexpected, given the observed frequencies of hyperuricemia (20 percent) and gout (about 5 percent) among patients with CPPD disease .
IMAGING FINDINGS — Imaging evidence for calcium pyrophosphate dihydrate (CPP) crystal deposition (CPPD) has traditionally relied upon plain film radiography, which reveals findings of cartilage calcification (see 'Cartilage calcification (chondrocalcinosis)' below). Degenerative changes in the joint are also frequently present, and certain radiographic findings in particular joints are characteristic of CPPD disease in those locations (see 'Degenerative changes' below and 'Other radiographic features in specific joints' below). Ultrasonographic findings that correlate with radiographic features of CPPD disease have also been described (see 'Ultrasonographic findings' below). Magnetic resonance imaging (MRI) is a less sensitive imaging modality for documenting CPP crystal deposition than plain film radiography, ultrasonography, or computed tomography (CT).
Plain film radiography
Cartilage calcification (chondrocalcinosis) — Radiographic evidence of calcium crystal deposition is the defining feature of this finding. CPP crystal deposits typically appear as punctate and linear radiodensities in articular cartilage (fibrocartilage and/or hyaline cartilage) (image 1), and, with lesser frequency, in ligaments, tendons, synovia, bursae, and joint capsules. Although deposits of carbonate-substituted calcium phosphate crystals may occasionally cause confusion, such deposits are usually faint and are irregularly contoured.
●Cartilage – Among affected fibrocartilages in CPPD disease are the menisci of the knee (usually bilaterally), the symphysis pubis, the triangular discs of the wrist joints, and the glenoid and acetabular labra. CPP crystal deposits in hyaline cartilage frequently appear as a radiopaque line paralleling the surface of the underlying bone.
●Joints – Larger joints, such as the knee, wrist, elbow, shoulder, and hip, are most frequently involved in CPPD disease, but almost any diarthrodial joint may be affected radiographically. Articular capsule or synovial calcification is often fainter and more diffuse than cartilage calcification.
Linear calcifications involving the Achilles tendon or plantar fascia are often seen in CPPD disease .
Degenerative changes — CPP crystal deposition is often associated with degenerative changes in joints, even in the absence of radiographic cartilage calcification. Characteristic degenerative changes in CPPD disease include subchondral cysts, osteophyte formation, and bone and cartilage fragmentation.
CPPD disease may also underlie the occurrence of radiographic features of osteoarthritis (OA) in joints not commonly affected by primary OA (see 'Diagnostic criteria' below and 'Differential diagnosis' below). Cartilage calcification that is apparent at the onset of degenerative changes or which occurs earlier in life than usual and without a pertinent vocational or avocational history is also characteristic of CPPD disease, whether in joints typical or atypical for OA.
Stress fractures or osteonecrosis (avascular necrosis), particularly in structures of the knee, may be consequences of or may be promoted by CPP crystal deposition.
Other radiographic features in specific joints — A variety of radiographic signs restricted to joints or regions are more or less characteristic of CPPD arthropathy, sometimes in addition to cartilage calcification (see 'Cartilage calcification (chondrocalcinosis)' above). The affected joints and findings include:
●Metacarpophalangeal (MCP) joints – Squared-off bone ends and hook-like osteophytes in the MCP joints, particularly if these changes are located in the second and third MCP joints. Such changes are especially common in hemochromatosis  and hemochromatosis-associated CPPD disease . (See "Clinical manifestations and diagnosis of hereditary hemochromatosis".)
●Wrist – Isolated or unusually extensive radiocarpal joint narrowing and/or navicular-lunate dissociation 
●Patellofemoral joints – Severe patellofemoral joint space degeneration, especially with a wrapped-around deformity of the patella on the femur. This finding is seen in hyperparathyroidism with or without CPP crystal deposition. Notching or erosion of the distal femoral cortex superior to the patella may also occur .
●Spine and pelvis – Axial skeleton changes, such as subchondral cysts in the small joints of the spine and in the sacroiliac joints, calcification of multiple intervertebral discs, and sacroiliac joint vacuum phenomena .
Crowned dens syndrome (CDS) (see 'Spinal involvement' above) can be identified by use of CT, which is the preferred modality to demonstrate its presence. Findings may include CPP (or hydroxyapatite) crystal masses at the atlanto-axial articulation and/or in the transverse ligament of the atlas and/or in the ligamentum flavum .
Ultrasonographic findings — The following findings on ultrasonography of articular and fibrocartilage may be indicative of the presence of deposits of CPP crystals [39,40]:
●A thin hyperechoic band paralleling the bone cortex and separated from it by a hypoechoic region representing cartilage. The resulting ultrasonographic appearance resembles the double contour sign (DCS) initially described in gout , but it often exhibits a thin, stippled appearance rather than the smooth pattern characteristic of gout.
●Small hyperechoic rounded amorphous shaped regions, often with acoustic shadowing, which are most often found in images of fibrocartilage of the wrist (image 2) and menisci of the knee, and in tendons.
●Nodular hyperechoic deposits in bursae and articular recesses.
●Hyperechoic lines of calcification running parallel to tendon fibers.
In contrast to urate crystal deposits in gout, CPP crystals often deposit within the substance of hyaline cartilage, providing a potentially attractive means to distinguish between these crystal deposition arthropathies.
Ultrasonography is a promising modality for clinical use in the diagnosis of CPPD disease and tracking the efficacy of CPPD disease therapies; however, further studies are warranted for: validation of ultrasound criteria unique to CPP crystal deposition, resolution of differences reported with regard to the sensitivity and specificity of the procedure for the diagnosis of CPPD disease [39,42-45], and comparison of imaging findings with corresponding histopathology as the gold standard.
Based upon the limited data available, ultrasonography appears to have sensitivity for aspiration-confirmed CPPD disease similar to that of plain film radiography when employed by experts in joint imaging . The utility of ultrasonography in diagnosing cartilage calcification outside of research centers, however, remains uncertain. Thus, we do not believe that the results of ultrasonographic study yet qualify as a diagnostic imaging criterion for CPPD disease.
DIAGNOSIS — The diagnosis of calcium pyrophosphate (CPP) deposition (CPPD) disease is largely based upon the demonstration of CPP crystals in tissue or synovial fluid and/or upon radiographic evidence of the disease (see 'Diagnostic evaluation' below and 'Diagnostic criteria' below). Generally, differences between the patterns of joint involvement in urate gout and acute CPP crystal arthritis are insufficient to permit definite diagnosis without demonstration of the specific crystal in the inflammatory joint effusion (picture 1).
A diagnosis of CPPD disease should be suspected in the patient (most often over age 65) with acute or subacute attacks of arthritis (particularly of the knee), arthritis similar in character to rheumatoid arthritis (RA) or osteoarthritis (OA), apparent CPP crystal deposition on radiograph, and in patients with other selected but less common presentations. (See 'Clinical manifestations' above.)
Diagnostic evaluation — Patients suspected of CPPD disease should undergo arthrocentesis and synovial fluid analysis of the affected joint and plain film radiography of the involved joint(s) (see 'Synovial fluid analysis' below and 'Diagnostic imaging' below). Radiographs of asymptomatic joints are sometimes also required.
Synovial fluid analysis — Arthrocentesis of an affected joint with synovial fluid analysis for CPP crystals should be promptly undertaken, if possible, to establish the diagnosis (see 'Synovial fluid findings' above). Synovial fluid analysis should also include a white blood cell count and differential, Gram stain, and culture to exclude alternative or concurrent diagnoses. Joint aspiration technique and synovial fluid analysis are described in detail separately. (See "Joint aspiration or injection in adults: Technique and indications" and "Synovial fluid analysis".)
Certain technical factors are important in the effort to identify CPP crystals in synovial fluid and tissue:
●Synovial fluid – Examination of synovial fluid for CPP crystals should be carried out promptly after joint aspiration, because successful identification of the crystals diminishes as the time elapsed between joint aspiration and microscopic examination increases. Another technical source of error is that positively birefringent material other than CPP crystals may be identified in joint fluid samples unless care is taken to clean and remove dust from the microscope slide and cover slip. Lack of attention to these details may contribute to reports of a relatively low specificity and sensitivity in identifying CPP crystals in synovial fluid, particularly when the observers carrying out the analysis are relatively inexperienced [46-48]. The European League Against Rheumatism (EULAR) recommendations for CPPD disease suggest refinement of an optimal protocol (including training) and standards for identification of CPP crystals in joint fluids .
●Synovial tissue – CPP crystals may also be present in synovial tissue, although tissue sections stained with hematoxylin and eosin (H&E) are less likely to demonstrate typical birefringent crystals, possibly due to the dissolution of crystalline CPP in the acid reagents. Use of alizarin red has been reported to preserve the crystals and reduces the likelihood of a false negative result, but false positive or negative staining may occur . A nonaqueous stain, such as Wright's, is another alternative to routine H&E staining for synovial tissue.
Diagnostic imaging — Plain radiographs of affected joints should be obtained to identify evidence of cartilage calcification (chondrocalcinosis) or other signs characteristic of CPPD crystal deposition disease. (See 'Plain film radiography' above.)
In patients in whom the diagnosis is suspected, but plain radiographs of pertinent affected joints have not shown evidence of cartilage calcification screening, radiographs of other frequently involved joints should be obtained, since the radiographic features of CPP crystal deposition disease have good specificity and sensitivity. We suggest that such radiographs should include:
●Anterior to posterior (AP) view of each knee
●AP view of the symphysis pubis
●Posterior to anterior (PA) views of each wrist
A surprising number of patients demonstrate cartilage calcification on radiographs and computed tomography (CT) performed for other than musculoskeletal evaluations at a variety of anatomic sites [50,51]. These deposits are often unrecognized or recognized but not reported by radiologists. Thus, reviewing films and CTs of the chest and pelvis may disclose unreported cartilage calcification.
Diagnostic criteria — The following diagnostic criteria and diagnostic classification for CPPD disease are based upon those initially proposed by McCarty and colleagues [2,3].
●Definite CPPD disease – A definite diagnosis of CPPD disease requires either:
•The presence of both positively (but weakly) birefringent crystals by compensated polarized light microscopy and typical cartilage or joint capsule calcification on radiograph examination of any typically involved joint OR
•The demonstration of CPP crystals in tissue or synovial fluid by definitive means (eg, radiograph diffraction, etc)
In clinical practice, definitive demonstration of crystals by radiograph diffraction powder pattern, electron microscopy, chemical analysis, or atomic force microscopy is only rarely achieved except in specialized research centers. Thus, the diagnosis is most commonly established by fulfilling the combination of radiographic and synovial fluid analysis criteria.
●Probable CPPD disease – A probable diagnosis of CPPD crystal disease occurs with either:
•The identification of positively (but weakly) birefringent crystals by compensated polarized light microscopy OR
•The presence of typical cartilage or joint capsule calcification on radiographic examination of any typically affected joint
The majority of patients thought to have CPPD disease by rheumatologists fall into the "probable disease" category. Given the state of management of this disorder, this fact does not appear to be a serious diagnostic quandary if alternative diagnostic possibilities are carefully considered and are acted upon appropriately.
●Possible CPPD disease – Fulfillment of either of the following criteria should raise the suspicion of possible CPPD disease and should prompt a direct evaluation aimed at fulfilling specific criteria for the disorder:
•Acute arthritis of large joints, especially the knees, OR
•Chronic arthritis which resembles OA, particularly if the involved joints are not typical for OA (wrists, metacarpophalangeal [MCP] joints, elbows, and shoulders) and if it is accompanied by acute, self-limited inflammatory attacks. As previously mentioned, distinction from OA is further suggested by radiographic features typical of CPPD arthropathy in individual joints and the spine (even in the absence of cartilage calcification); severe and progressive joint degeneration, especially with prominent subchondral cyst formation; and tendon calcifications apparent on radiographs.
When the arthritic process occurs in joints typical for osteoarthritis (such as the interphalangeal joints of the hands, the first carpometacarpal joints, the knees, or the bunion joints), even radiograph or joint fluid evidence of CPP crystal deposition does not permit an unequivocal diagnosis. However, an etiologic or accelerating role for CPP crystal deposition in joint degeneration seems reasonable when radiographic calcification is present early in the course of degeneration, particularly if the degenerative changes also involve joints atypical for OA (wrists, MCP joints, elbows, and shoulders) in the absence of a preceding history of joint trauma or of vocational or avocational stress.
The criteria for possible CPPD disease are aimed at calling attention to the possibility of the disorder and at stimulating the search for more definitive criteria. They are not intended to attach a diagnostic label to an individual patient.
DIFFERENTIAL DIAGNOSIS — The differential diagnosis of each of the presentations of calcium pyrophosphate dihydrate (CPP) crystal deposition (CPPD) disease (see 'Clinical manifestations' above) is addressed separately below:
●Asymptomatic cartilage calcification – CPPD without symptoms ("asymptomatic CPPD") is typically discovered when chondrocalcinosis is incidentally noted on radiographs (see 'Cartilage calcification (chondrocalcinosis)' above). Articular or fibrocartilage calcification may be an indication of an underlying, potentially treatable disorder for which screening tests may be valuable. (See 'Post-diagnostic evaluation for associated diseases' below.)
●Acute arthritis – Acute CPP crystal arthritis closely resembles gout in its clinical features (hence the term "pseudogout"), including acute or subacute attacks in a single or only a few joints, with prominent signs of local inflammation and sometimes systemic signs such as leukocytosis and elevated acute phase reactants. In addition, trauma, surgery, or severe medical illness may provoke acute attacks in both of these crystal-induced disorders. Despite these similarities, the joints most commonly involved in an acute episode differ between gout and pseudogout. The knee is affected in over 50 percent of all acute attacks of pseudogout, whereas the first metatarsophalangeal (MTP) joint is more frequently involved in gout. The distinction between these conditions is made based upon identification of crystals under polarizing light microscopy that are characteristic of one or the other condition. (See "Clinical manifestations and diagnosis of gout".)
The broader differential diagnosis of an acute monoarthritis is discussed in detail separately; however, infection, trauma, bleeding, and other crystal-associated diseases (eg, gout) are among the important disorders to consider. These conditions can usually be distinguished from pseudogout by synovial fluid analysis and joint radiography, although the presence of CPP crystals in joint fluid does not exclude infection. Thus, Gram stain, as well as joint fluid and blood cultures, is generally necessary to exclude an infectious etiology. (See "Overview of monoarthritis in adults" and "Septic arthritis in adults" and "Synovial fluid analysis".)
●Chronic inflammatory arthritis – Chronic CPP inflammatory arthritis ("pseudo-rheumatoid arthritis [RA]") may present in a similar fashion to other forms of chronic inflammatory arthritis, particularly RA, and must be distinguished from RA and other causes of polyarthralgia or polyarthritis. In chronic CPPD inflammatory arthritis, the multiple inflamed joints typically flare independently of one another, unlike in RA, where multiple joints usually flare and improve in parallel. Radiographic changes in chronic CPPD inflammatory arthritis are more typical of osteoarthritis (OA) than RA, despite the pattern of clinical joint involvement suggestive of the latter disorder (see 'Imaging findings' above). The diagnostic approach to adults with polyarticular joint pain is presented in detail elsewhere. (See "Evaluation of the adult with polyarticular pain".)
Diagnostic confusion between pseudo-RA and RA may arise when inflammation is more or less symmetrical and, particularly, when rheumatoid factor is detectable (as is the case in a substantial proportion of normal older individuals). Generally, the identification of CPP crystals and radiographic changes typical of CPPD disease will help distinguish the disorders. However, unlike gout, which is negatively associated with RA (occurring less often together than would be predicted from the prevalence of each condition in the population), CPPD disease and RA appear to coexist about as frequently as would be expected from the individual prevalence of the two disorders. It is important to distinguish between the coexistence of RA and CPPD disease and the expression of one masquerading as the other because of the substantial difference in management between these conditions. (See "Diagnosis and differential diagnosis of rheumatoid arthritis".)
A rare subtype of chronic CPP inflammatory arthritis is characterized by prominent systemic features such as leukocytosis, fever, mental confusion, and inflammatory polyarthritis  In older adults, this clinical presentation can closely mimic systemic sepsis, from which it is distinguishable both by the presence of CPP crystals in synovial fluid from affected joints and by reversal of the clinical features during antiinflammatory drug treatment.
Other forms of subacute and chronic inflammatory arthritis, such as peripheral spondyloarthritis, including reactive arthritis and arthritis associated with inflammatory bowel disease, as well as psoriatic arthritis, can generally be distinguished from CPPD-related arthritis by the presence of other clinical features typical of these disorders and the absence of CPP crystals from synovial fluid and of radiographic findings of cartilage calcification. (See "Reactive arthritis" and "Clinical manifestations and diagnosis of psoriatic arthritis" and "Clinical manifestations and diagnosis of arthritis associated with inflammatory bowel disease and other gastrointestinal diseases".)
●Chronic degenerative arthritis – In OA with CPPD (pseudo-OA), the findings on clinical examination of individual joints do not typically differ from those observed in OA, and the symptoms and radiographic features include those of OA. However, patients with OA with CPPD often exhibit changes in joints not typically affected in patients with OA alone, and findings of cartilage calcification are often present before there is significant degenerative joint disease, although patients may not have been evaluated at this stage of illness. In addition to the atypical distribution, other features on examination which should alert the clinician to possible CPPD disease include contractures of involved joints and valgus deformities of the knees. The clinical manifestations and diagnosis of OA are discussed in detail elsewhere. (See "Clinical manifestations and diagnosis of osteoarthritis".)
●Severe joint degeneration – Pseudo-neuropathic joint disease associated with CPP crystal deposition may resemble neuropathic joint destruction radiographically, with severe degenerative changes and disorganization of the bony structures in the joint. Unlike pseudo-neuropathic joint disease, true neuropathic arthropathy is characterized by loss of joint position sensation and sometimes by loss of other sensory function, such as pain and temperature sensation. Disorders associated with neuropathic arthropathy (Charcot joint) include diabetes mellitus (most common), tabes dorsalis, syringomyelia, and spinal cord injuries, and their presence should increase suspicion of this condition. The presence of CPP crystals, by contrast, is consistent with pseudo-neuropathic joint disease instead. (See "Diabetic neuropathic arthropathy".)
●Nerve or spinal cord compression – Mass effects from collections of CPPD crystals and associated soft tissue must be distinguished from other calcifying or ossifying mass lesions. An intraspinal mass with punctate soft tissue calcification suggests chondrosarcoma, and histopathologic examination of the resected tissue may be required to exclude a malignant neoplasm  (see "Spinal cord tumors", section on 'Sarcomas'). Monosodium urate tophi may rarely cause neurologic compressive syndromes. (See "Clinical manifestations and diagnosis of gout".)
POST-DIAGNOSTIC EVALUATION FOR ASSOCIATED DISEASES — Patients with a diagnosis of calcium pyrophosphate dihydrate (CPP) crystal deposition (CPPD) disease, even if asymptomatic, may also have one of a number of associated disorders, including hemochromatosis, hyperparathyroidism, hypomagnesemia, hypophosphatasia, and familial hypocalciuric hypercalcemia (table 1) . The conditions associated with CPPD disease are reviewed in detail separately. (See "Pathogenesis and etiology of calcium pyrophosphate crystal deposition (CPPD) disease".)
Given these disease associations, we suggest that patients diagnosed with CPP crystal deposition undergo the following serum screening studies :
•Iron and transferrin – In addition to ferritin, iron and transferrin studies are also useful, given the limitations of interpreting the ferritin level alone. In hemochromatosis, the ferritin level may be falsely normal after bleeding, classically in women with heavy menses, anyone after an occult and slow gastrointestinal bleed, or in celiac disease with iron absorption problems. Additionally, when an elevated ferritin level is present as an acute phase reactant or in liver disease, it is helpful to have the iron study results available without having to redraw the patient's blood.
A search for these associated metabolic conditions is particularly fruitful when CPPD is diagnosed at a young age. Familial forms of CPPD disease may also occur early in life.
Appropriate further evaluation should be undertaken if one or more of these values are abnormal. In patients over 60 years of age, serum magnesium and alkaline phosphatase determinations may be omitted because chronic hypomagnesemia and hypophosphatasia present earlier in life. Despite the value of establishing and treating the non-musculoskeletal system organ-specific abnormalities associated with the presence of some of the disorders associated with CPPD disease, the results of specific treatments directed at the respective associated disorders, such as reversal of the iron overload in hemochromatosis or the parathyroid hormone excess in hyperparathyroidism, have been ineffective in halting or reversing the crystal deposition disorder.
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Gout and calcium pyrophosphate deposition disease".)
INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.
Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)
●Basics topics (see "Patient education: Calcium pyrophosphate deposition disease (The Basics)")
●Beyond the Basics topics (see "Patient education: Calcium pyrophosphate crystal deposition (CPPD) disease (Beyond the Basics)")
SUMMARY AND RECOMMENDATIONS
●The names traditionally used for the calcium pyrophosphate crystal-related disorders include: pseudogout, for the acute attacks of inflammatory arthritis caused by calcium pyrophosphate crystal (CPP) deposition; chondrocalcinosis, for the radiographic calcification in hyaline and/or fibrocartilage; and pyrophosphate arthropathy, for the radiographic or joint abnormalities accompanying calcium pyrophosphate dihydrate crystal deposition (CPPD) disease. (See 'Terminology' above.)
●CPPD disease is a disease of older adults, and radiographic surveys demonstrate an age-related increase in the prevalence of articular cartilage calcification. There is no major sex predominance, although attacks of acute CPP crystal arthritis (often termed pseudogout) occur more frequently in men, while osteoarthritis (OA) with CPP deposition is more common in women. (See 'Epidemiology' above.)
●The clinical spectrum of CPPD disease is broad, even though the majority of patients are probably asymptomatic. The range of presentations, named as proposed by a European League Against Rheumatism (EULAR) task force (with alternative and traditional terms in parentheses) includes (see 'Clinical manifestations' above and 'Terminology' above):
•Asymptomatic CPPD disease ("asymptomatic CPPD") (see 'Asymptomatic CPPD disease' above)
•Acute CPP crystal arthritis ("pseudogout") (see 'Acute CPP crystal arthritis' above)
•Chronic CPP crystal inflammatory arthritis ("pseudo-RA") (see 'Chronic CPP crystal inflammatory arthritis' above)
•OA with CPPD, with or without superimposed acute attacks ("pseudo-OA") (see 'Osteoarthritis with CPPD' above)
•Severe joint degeneration (pseudo-neuropathic joint disease) (see 'Severe joint degeneration' above)
●Synovial fluid analysis in CPPD disease is characterized by the presence of weakly positively birefringent CPP crystals that may be detected by compensated polarized light microscopy. Phagocytosed crystals within polymorphonuclear leukocytes are virtually always present in inflamed joints during an acute attack of CPP crystal arthritis (picture 1). Examination of synovial fluid for CPP crystals should be carried out promptly after joint aspiration, because successful identification of the crystals diminishes as the time elapsed between joint aspiration and microscopic examination increases. The total synovial fluid leukocyte concentration is typically 15,000 to 30,000 per mm3, 90 percent of which are neutrophils. (See 'Synovial fluid findings' above and 'Synovial fluid analysis' above.)
●Plain film radiographic calcium crystal deposition in fibrocartilage, in or hyaline or articular cartilage, called cartilage calcification (chondrocalcinosis), is a common accompaniment of symptomatic CPPD but frequently exists in asymptomatic individuals. CPP crystal deposits typically appear as punctate and linear radiodensities in cartilage (image 1) and, with lesser frequency, in ligaments, tendons, synovium, and joint capsules. Even in the absence of demonstrable cartilage calcification, CPP crystal deposition is often detectable in patients with degenerative changes in joints. Ultrasonographic findings that correlate with radiographic features of CPPD disease have also been described (image 2). (See 'Imaging findings' above and 'Plain film radiography' above and 'Ultrasonographic findings' above.)
●A diagnosis of CPPD disease should be suspected in the patient (most often over age 65) with acute or subacute attacks of arthritis (particularly of the knee), arthritis similar in character to RA or OA, apparent CPP crystal deposition on radiograph, or other less common presentations. Establishing the diagnosis is largely based upon the demonstration of CPP crystals in tissue or synovial fluid and/or upon radiographic evidence of the disease. Screening radiographs of other frequently involved joints should be obtained in all patients in whom the diagnosis is suspected but in whom radiographs of pertinent affected joints have not yielded a definitive diagnosis. (See 'Clinical manifestations' above and 'Diagnosis' above and 'Diagnostic criteria' above.)
●Patients with a diagnosis of CPPD disease may also have one of a number of associated disorders, including hemochromatosis, hyperparathyroidism, hypomagnesemia, hypophosphatasia, and familial hypocalciuric hypercalcemia (table 1). Given these disease associations, patients diagnosed with CPP crystal deposition, even if asymptomatic and especially if less than 50 years of age, should undergo the following serum screening studies: calcium, phosphorus, magnesium, alkaline phosphatase, ferritin, iron, and transferrin. (See 'Post-diagnostic evaluation for associated diseases' above.)
●The differential diagnosis of CPPD disease is broad and differs for each of the potential clinical presentations. Major considerations include gout and septic arthritis, RA, and OA. (See 'Differential diagnosis' above.)
ACKNOWLEDGMENT — The editorial staff at UpToDate would like to acknowledge Lawrence Ryan, MD, who contributed to an earlier version of this topic review.
- McCarty DJ. Calcium pyrophosphate dihydrate crystal deposition disease--1975. Arthritis Rheum 1976; 19 Suppl 3:275.
- Rosenthal AK, Ryan LM, McCarty DJ. Calcium pyrophosphate crystal deposition disease, pseudogout, and articular chondrocalcinosis. In: Arthritis and Allied Conditions, 15th, Koopman WJ, Moreland LW (Eds), Lippincott Williams & Wilkins, Philadelphia 2005. p.2373.
- Rosenthal AK. Pseudogout: Presentation, natural history, and associated conditions. In: Crystal-Induced Arthropathies: Gout, Pseudogout, and Apatite-Associated Syndromes, Wortmann RL, Schumacher HR Jr, Becker MA, Ryan LM (Eds), Taylor and Francis Group, New York 2006. p.99.
- Zhang W, Doherty M, Bardin T, et al. European League Against Rheumatism recommendations for calcium pyrophosphate deposition. Part I: terminology and diagnosis. Ann Rheum Dis 2011; 70:563.
- Rosenthal AK, Ryan LM. Crystal arthritis: calcium pyrophosphate deposition-nothing 'pseudo' about it! Nat Rev Rheumatol 2011; 7:257.
- Rosenthal AK, Ryan LM. Calcium Pyrophosphate Deposition Disease. N Engl J Med 2016; 374:2575.
- Neame RL, Carr AJ, Muir K, Doherty M. UK community prevalence of knee chondrocalcinosis: evidence that correlation with osteoarthritis is through a shared association with osteophyte. Ann Rheum Dis 2003; 62:513.
- Felson DT, Naimark A, Anderson J, et al. The prevalence of knee osteoarthritis in the elderly. The Framingham Osteoarthritis Study. Arthritis Rheum 1987; 30:914.
- Ellman MH, Levin B. Chondrocalcinosis in elderly persons. Arthritis Rheum 1975; 18:43.
- Wilkins E, Dieppe P, Maddison P, Evison G. Osteoarthritis and articular chondrocalcinosis in the elderly. Ann Rheum Dis 1983; 42:280.
- O'Duffy JD. Clinical studies of acute pseudogout attacks: comments on prevalence, predispositions, and treatment. Arthritis Rheum 1976; 19 Suppl 3:349.
- Dieppe PA, Alexander GJ, Jones HE, et al. Pyrophosphate arthropathy: a clinical and radiological study of 105 cases. Ann Rheum Dis 1982; 41:371.
- Masuda I, Ishikawa K. Clinical features of pseudogout attack. A survey of 50 cases. Clin Orthop Relat Res 1988; :173.
- Bilezikian JP, Connor TB, Aptekar R, et al. Pseudogout after parathyroidectomy. Lancet 1973; 1:445.
- Luzar MJ, Altawil B. Pseudogout following intraarticular injection of sodium hyaluronate. Arthritis Rheum 1998; 41:939.
- Malnick SD, Ariel-Ronen S, Evron E, Sthoeger ZM. Acute pseudogout as a complication of pamidronate. Ann Pharmacother 1997; 31:499.
- Sandor V, Hassan R, Kohn E. Exacerbation of pseudogout by granulocyte colony-stimulating factor. Ann Intern Med 1996; 125:781.
- Pullman-Mooar S, Mooar P, Sieck M, et al. Are there distinctive inflammatory flares after hylan g-f 20 intraarticular injections? J Rheumatol 2002; 29:2611.
- Bong D, Bennett R. Pseudogout mimicking systemic disease. JAMA 1981; 246:1438.
- Derfus BA, Kurian JB, Butler JJ, et al. The high prevalence of pathologic calcium crystals in pre-operative knees. J Rheumatol 2002; 29:570.
- Bennett RM, Mall JC, McCarty DJ. Pseudogout in acute neuropathic arthropathy. A clue to pathogenesis? Ann Rheum Dis 1974; 33:563.
- Gerster JC, Vischer TL, Fallet GH. Destructive arthropathy in generalized osteoarthritis with articular chondrocalcinosis. J Rheumatol 1975; 2:265.
- Richards AJ, Hamilton EB. Destructive arthropathy in chondrocalcinosis articularis. Ann Rheum Dis 1974; 33:196.
- Reginato AJ, Hollander JL, Martinez V, et al. Familial chondrocalcinosis in the Chiloe Islands, Chile. Ann Rheum Dis 1975; 34:260.
- Cabre P, Pascal-Moussellard H, Kaidomar S, et al. Six cases of cervical ligamentum flavum calcification in Blacks in the French West Indies. Joint Bone Spine 2001; 68:158.
- Muthukumar N, Karuppaswamy U. Tumoral calcium pyrophosphate dihydrate deposition disease of the ligamentum flavum. Neurosurgery 2003; 53:103.
- Yayama T, Kobayashi S, Sato R, et al. Calcium pyrophosphate crystal deposition in the ligamentum flavum of degenerated lumbar spine: histopathological and immunohistological findings. Clin Rheumatol 2008; 27:597.
- Aouba A, Vuillemin-Bodaghi V, Mutschler C, De Bandt M. Crowned dens syndrome misdiagnosed as polymyalgia rheumatica, giant cell arteritis, meningitis or spondylitis: an analysis of eight cases. Rheumatology (Oxford) 2004; 43:1508.
- Godfrin-Valnet M, Godfrin G, Godard J, et al. Eighteen cases of crowned dens syndrome: Presentation and diagnosis. Neurochirurgie 2013; 59:115.
- Gerster JC, Lagier R, Boivin G, Schneider C. Carpal tunnel syndrome in chondrocalcinosis of the wrist. Clinical and histologic study. Arthritis Rheum 1980; 23:926.
- Bjelle A, Crocker P, Willoughby D. Ultra-microcrystals in pyrophosphate arthropathy. Crystal identification and case report. Acta Med Scand 1980; 207:89.
- Pereira ER, Brown RR, Resnick D. Prevalence and patterns of tendon calcification in patients with chondrocalcinosis of the knee: radiologic study of 156 patients. Clin Imaging 1998; 22:371.
- Adamson TC 3rd, Resnik CS, Guerra J Jr, et al. Hand and wrist arthropathies of hemochromatosis and calcium pyrophosphate deposition disease: distinct radiographic features. Radiology 1983; 147:377.
- Martel W, Champion CK, Thompson GR, Carter TL. A roentgenologically distinctive arthropathy in some patients with the pseudogout syndrome. Am J Roentgenol Radium Ther Nucl Med 1970; 109:587.
- Resnick D, Niwayama G. Carpal instability in rheumatoid arthritis and calcium pyrophosphate deposition disease. Pathogenesis and roentgen appearance. Ann Rheum Dis 1977; 36:311.
- Lagier R. Rare femoral erosions and osteoarthrosis of the knee associated with chondrocalcinosis. A histological study of this cortical remodelling. Virchows Arch A Pathol Anat Histol 1974; 364:215.
- Martel W, McCarter DK, Solsky MA, et al. Further observations on the arthropathy of calcium pyrophosphate crystal deposition disease. Radiology 1981; 141:1.
- Matsumura M, Hara S. Images in clinical medicine. Crowned dens syndrome. N Engl J Med 2012; 367:e34.
- Frediani B, Filippou G, Falsetti P, et al. Diagnosis of calcium pyrophosphate dihydrate crystal deposition disease: ultrasonographic criteria proposed. Ann Rheum Dis 2005; 64:638.
- Filippucci E, Scirè CA, Delle Sedie A, et al. Ultrasound imaging for the rheumatologist. XXV. Sonographic assessment of the knee in patients with gout and calcium pyrophosphate deposition disease. Clin Exp Rheumatol 2010; 28:2.
- Thiele RG, Schlesinger N. Diagnosis of gout by ultrasound. Rheumatology (Oxford) 2007; 46:1116.
- Foldes K. Knee chondrocalcinosis: an ultrasonographic study of the hyalin cartilage. Clin Imaging 2002; 26:194.
- Filippou G, Frediani B, Gallo A, et al. A "new" technique for the diagnosis of chondrocalcinosis of the knee: sensitivity and specificity of high-frequency ultrasonography. Ann Rheum Dis 2007; 66:1126.
- Filippucci E, Riveros MG, Georgescu D, et al. Hyaline cartilage involvement in patients with gout and calcium pyrophosphate deposition disease. An ultrasound study. Osteoarthritis Cartilage 2009; 17:178.
- Löffler C, Sattler H, Peters L, et al. Distinguishing gouty arthritis from calcium pyrophosphate disease and other arthritides. J Rheumatol 2015; 42:513.
- Gordon C, Swan A, Dieppe P. Detection of crystals in synovial fluids by light microscopy: sensitivity and reliability. Ann Rheum Dis 1989; 48:737.
- Schumacher HR Jr, Sieck MS, Rothfuss S, et al. Reproducibility of synovial fluid analyses. A study among four laboratories. Arthritis Rheum 1986; 29:770.
- Hasselbacher P. Variation in synovial fluid analysis by hospital laboratories. Arthritis Rheum 1987; 30:637.
- Yamakawa K, Iwasaki H, Masuda I, et al. The utility of alizarin red s staining in calcium pyrophosphate dihydrate crystal deposition disease. J Rheumatol 2003; 30:1032.
- Parperis K, Carrera G, Baynes K, et al. The prevalence of chondrocalcinosis (CC) of the acromioclavicular (AC) joint on chest radiographs and correlation with calcium pyrophosphate dihydrate (CPPD) crystal deposition disease. Clin Rheumatol 2013; 32:1383.
- Shirazian H, Chang EY, Wolfson T, et al. Prevalence of sternoclavicular joint calcium pyrophosphate dihydrate crystal deposition on computed tomography. Clin Imaging 2014; 38:380.
- Ishida T, Dorfman HD, Bullough PG. Tophaceous pseudogout (tumoral calcium pyrophosphate dihydrate crystal deposition disease). Hum Pathol 1995; 26:587.
- Jones AC, Chuck AJ, Arie EA, et al. Diseases associated with calcium pyrophosphate deposition disease. Semin Arthritis Rheum 1992; 22:188.